This invention relates to the condensation of zinc vapor produced in thermal reduction processes, for example in the zinc blast furnace process.
In the zinc blast furnace process, zinc vapor leaving the top of the furnace is condensed by passing it through a lead-splash condenser where the zinc vapor is contacted with an intense spray of molten lead droplets. A lead-splash condenser, when viewed schematically, approximates to a rectangular chamber provided at one end with a gas inlet duct of large cross-sectional area, this duct usually sloping downwardly to the condenser from the top of the furnace shaft, and provided at the other end with a gas outlet duct which includes a vertical or near vertical stack portion.
The intense spray of molten lead within the lead-splash condenser is generated in some suitable fashion, for example by a series of rotatable impellers immersed in a pool or pools of molten lead, in a number of separate stages.
The zinc is condensed by means of the spray of molten lead and the molten lead containing the condensed zinc flows from the condenser via an underflow baffle into a sump, known as a pump sump, from which it is transferred by a suitable pump into an elongated cooled launder. The lead is partially cooled during its passage through the launder, for example by immersion coolers, and the flowing metal, from being a one phase solution of zinc in lead, on transfer into the launder from the sump becomes a two phase system of (1) zinc containing a little lead on top of (2) lead still containing some zinc. This two phase lead/zinc system of molten metals is admitted to a separator and zinc is recovered therefrom. Cooled lead is returned to the condenser by a short launder, again via an underflow baffle.
The sensible heat of the input gases to the condenser is partially transferred to the molten lead and a thermal balance is established in the system. The factors which determine the temperatures at each end of the condenser are the gas inlet temperature and the temperature of the lead leaving the separator. There is little latitude to vary these temperatures since they are determined by the requirements for efficient operation of the blast furnace shaft and of the separator system.
The efficiency of this type of condenser system may be determined by measurement of the quantity of zinc carried out of the condenser by the gases. In conventional systems, up to about 9% of the zinc vapor entering the condenser is not recovered, and thus the condensation efficiency of such condensers may be as low as 91%.